The invention is directed to systems and methods for measuring and controlling wavelength, and more particularly to such systems and methods that accurately measure and control the output wavelength of CW and pulsed lasers.
Measuring and controlling a laser wavelength accurately over a large continuous wavelength range at low continuous or pulsed input power and/or at high pulse repetition rate is a difficult and important problem in many areas of technology, including excimer laser microlithography and wavelength division multiplexed (WDM) fiber optic communication systems. Previously disclosed approaches for measuring laser wavelengths suffer from one or more limitations, including poor sensitivity, difficult calibration, long measurement times, or the physical size of the equipment.
At present there exist at least four methods for measuring laser wavelength: grating spectrometers, etalon-based wavelength meters, and scanning Michelson interferometers, as well as a recently introduced phase sensitive wavelength meter. While all four methods can be used to measure continuous lasers, only grating spectrometers and etalon-based wavelength meters have been found acceptable for pulsed laser applications.
The resolution of a grating spectrometer is equal to the product of the diffraction order number m and the number of grooves N:
xcex/xcex94xcex=mNxe2x80x83xe2x80x83(1)
Equation (1) implies that the resolving power of the grating is equal to the number of wavelengths in the path difference between rays that are diffracted in the direction of the center wavelength of the laser line from the extreme ends of the gratings. In laser wavelength metrology it is often required to only locate the center of the laser spectrum with extreme accuracy, not to resolve closely spaced individual spectral lines. Still, in order to achieve the required linewidth line center localization of 0.1 pm, or better, very large grating spectrometers are required. In addition, the key to accurate wavelength measurement is the incorporation of very accurate and stable calibration sources into the spectrometer. Stable laser sources of known wavelength, such as frequency doubled Argon Ion laser light at 244 nm, or stabilized Helium-Neon lasers at 632.99 nm may be used. Another technique uses known absorption lines that are backlit by a broad band light source and the resulting absorption spectrum may impinge on the recording plane of the spectrometer onto a charge coupled device (CCD) or photodiode array detector.
Etalon-based wavelength meters are significantly smaller than high-resolution grating based spectrometers, and normally have higher resolution as well. A fixed Fabry-Perot etalon is composed of parallel, high reflectivity, mirrors separated by a defined spacing. Other types of etalons such as Fizeau or scanning Fabry-Perot etalons maybe used, however, since many of their performance characteristics are the same as fixed Fabry-Perot etalons they will not be reviewed here. We review the performance characteristics of the Fabry-Perot etalon based wavelength meter as an example of this category of wavelength meters. If the Fabry-Perot etalon is illuminated by collimated light, only a very narrow wavelength will be in resonance with the cavity and be transmitted. This geometry is not generally used in wavelength meter applications. If a diverging beam illuminates the etalon, transmission occurs only at angles of incidence such that the path length between mirrors is an integral number of incident wavelengths. This geometry is routinely used in laser wavelength meters.
An Etalon is characterized by a figure of merit called its finesse F which equals:                     𝔍        =                  π          ⁢                      xe2x80x83                    ⁢                                    R                              1                -                R                                                                        (        2        )            
Where R is the reflectivity of each mirror, assuming both cavity mirrors have the same reflectivity. For Fabry-Perot etalons illuminated with a diverging beam the resolving power xcex94xcex/xcex equals:                               λ          λ                =                  0.97          *          m          *          𝔍                                    (        3        )            
where m is the order number representing an integer number of wavelengths that separate the mirrors. At near normal incidence m≈2nxcex/xcex, where n is the refractive index of the medium between the mirrors at the wavelength X, and h is the physical separation of the mirrors. In order to use an etalon properly one must know the spacing between the mirrors to within an order number, or else there is ambiguity in determination of wavelength. The wavelength range between consecutive orders is called xe2x80x9cFree Spectral Rangexe2x80x9d (FSR) of the etalon, where:
FSR=xcex/m=xcex2/2nxcexxe2x80x83xe2x80x83(4)
Thus as can be seen from equation (4) the FSR decreases as the separation between the mirrors h increases. The resolution of the etalon also increases as the FSR increases. It is known that etalons themselves and the optics associated with an etalon-based wavelength meter are not absolutely stable. Therefore, the etalon spacing and angles of incidence of the laser beam must be continuously calibrated either with a stable reference laser, such as a HeNe laser, or with a backlit absorption cell having well-calibrated absorption lines. The calibration light should be well reflected by the etalon mirrors, and with adequate, such that the resolution of the reference lines is equivalent to that of the unknown laser lines. The spectra of the reference laser and of the excimer laser are both recorded by an array detector, usually a charge-coupled device (CCD), or a photodiode array. Ideally, a single array is used that is sensitive to both the reference and laser wavelengths to be measured.
A single etalon may be used to improve the measurement accuracy of the laser wavelength to 1 part in 50-100 of the Etalon""s FSR. It the laser wavelength is not known to the accuracy of about xc2xd the FSR, and then one or more additional etalons of reduced resolution can be used to more accurately estimate the wavelength before finally determining the laser wavelength with the etalon having the longest spacing.
Both spectrometer and etalon based wavelength measurement techniques suffer from serious drawbacks in wavelength measurement. Both techniques can use array detectors to measure an excimer laser spectrum. Array detectors can be read out at the 1-4 kHz sample rates required, but are inherently noisy during high speed scanning operation. This results in reduced signal detectivity and thus poor accuracy at higher data rates. Slower data rates result in a better signal-to-noise ratio, but may not properly record all the laser pulses. Spectrometers of adequate resolution for high-resolution laser wavelength measurement are large instruments, have very large gratings, are difficult to absolutely calibrate. Etalon spectrometers are inefficient. In transmission, light at only the correct angle produces interference orders. Therefore only about 1-5% of the incident light is used in measuring wavelength. If more than one etalon is required, due to inadequate initial knowledge of the laser wavelength, the efficiency drops rapidly as the incident power must be divided into several beams.
High-resolution scanning Michelson wavelength meters are often used to measure the wavelength of continuous or very high (greater than megahertz) repetition rate lasers. They cannot be used to measure pulsed lasers of low repetition rate (kHz). In a scanning Michelson interferometer the laser to be measured is split into 2 paths by a beamsplitter. At the end of each path, called an arm of the interferometer, is a reflecting mirror that directs the beams back to the beamsplitter. The 2 beams are recombined and the signal is recorded on a photodiode. The path length of one arm of the interferometer is continuously varied and a time sequence of the intensity of the two interfering beams is recorded by the digitized output of the photodiode. If the path length of the light in each arm differs by an integral number of wavelengths of the reference beam than the two beams will combine constructively. If the path length differs by xc2xd of a wavelength, the beams will interfere destructively. The laser wavelength is determined by either counting the number of interference peaks (called fringes), and measuring the final partial fringe in the scanning interferometer or by taking the Fourier transform of the entire interferometer scan. The actual measure of displacement is usually made by counting the fringes in a parallel or collinear laser of know wavelength such as a Helium Neon laser. Commercial systems are marketed that measure wavelength to 0.1 pm. However, these systems do not work with pulsed lasers, measurement time is long xe2x88x920.1-1.0 second per measurement and are limited in sensitivity to about 0.1 xcexcW of input power. U.S. Pat. No. 6,043,883 discloses another type of wavelength meter which uses a circularly polarized beam impinging on a birefringent crystal, referred to as retardation plate, with orthogonal polarizations aligned with the ordinary and extraordinary axes of the crystal. Each polarization direction propagates at a different velocity in the crystal based-upon the refractive index difference between the two axes. Thus as the beam propagates its polarization state can be described as elliptical. An analyzer at the output of the crystal separates out the polarization state of the elliptical light. The wavelength can be determined from the ratio of the intensities at the two polarizations and the thickness of the crystal. However, the refractive index of most birefringent crystals tends to have strong temperature dependence and the axes of the crystal need to be precisely aligned relative to the laser beam. Accordingly, the temperature of the crystal must be taken into account so that the temperature variations in the refractive index and physical dimensions of the crystal can be compensated for in the wavelength determination.
It would be desirable to provide a more accurate and less complex system and method for wavelength measurements as well as a wavelength meter that is more compact than conventional systems.
The invention is directed to a laser wavelength meter that measures a phase difference between two orthogonally circularly polarized laser beams derived from a single laser beam. The polarized beams propagate along two different optical paths of a polarizing beamsplitter that differ by a known path length difference, and are then recombined. The orthogonal polarizations of a circularly polarized reference laser of known wavelength, such as a He-Ne laser, are separated and combined along the same path as the laser of unknown wavelength. Knowledge of the reference laser wavelength and the phase difference built up by the reference laser allows calculation of the path length difference. The so determined path length difference is then used to calculate the unknown wavelength of the laser under test from a measurement of the phase difference built up by the laser under test. The longer the optical path length, the greater the accuracy with which the wavelength of the unknown laser can be determined.
According to one aspect of the invention, a wavemeter for measuring an unknown wavelength of an optical beam includes a reference beam of a known wavelength having two orthogonally circularly polarization directions, a polarizing beam splitter receiving the orthogonally polarized reference beam and defining-an interferometer with a difference in an optical path length between the two orthogonally circularly polarized directions, and a first analyzer receiving from the polarizing beam splitter a combined reference beam with the path length difference, the first analyzer determining the path length difference from an intensity measurement of the two orthogonally circularly polarization directions of the combined reference beam. The wavemeter further includes a polarizer converting the optical beam of the unknown wavelength into two orthogonally circularly polarized beams, with the two orthogonally circularly polarized beams of the unknown wavelength traversing in the polarizing beam splitter an optical path that is substantially identical to an optical path of the reference beam in the polarizing beam splitter, and a second analyzer that receives from the polarizing beam splitter a combined beam of the unknown wavelength with the path length difference, the second analyzer determining the unknown wavelength from an intensity measurement of the two orthogonally circularly polarization directions of the combined beam of the unknown wavelength and the path length difference determined by the reference beam.
Embodiments of the invention may include one or more of the following features. The first and second analyzer can include photodiodes that measure the intensity of the different polarized beams. At least one chopper may be provided that alternatingly interrupts the reference beam and the optical beam of unknown wavelength. In this case, the first and second analyzer can be the same analyzer. The path length difference of the interferometer can be determined by a length of the polarizing beam splitter. The polarizing beam splitter can also be implemented as a Wollaston prism, in which case the path length difference of the interferometer is determined by a step height difference of a stepped reflector.
The wavemeter may include more than one polarization beam splitter having different free spectral ranges (FSR), wherein each of the polarization beam splitters receive a portion of the reference beam and the beam of the unknown wavelength and has associated therewith at least one analyzer receiving from the corresponding polarizing beam splitter a combined reference beam and a combined beam of the unknown wavelength with a path length difference characteristic of the polarizing beam splitter, each of the analyzers determining the unknown wavelength with an accuracy depending on the FSR of the corresponding polarization beam splitter.
According to another aspect of the invention, a method is described for determining an unknown wavelength of an optical beam. The method includes propagating in an polarizing beam splitter two orthogonally circularly polarized beams produced from a reference beam, the two orthogonally circularly polarized beams traversing different optical paths having different path lengths; determining a difference in the optical path lengths for the reference beam;. generating from the optical beam of unknown wavelength two orthogonally circularly polarized beams; propagating in the polarizing beam splitter the two orthogonally circularly polarized beams produced from the optical beam along the different optical paths traversed by the reference beam; determining a phase shift between the propagated two orthogonally circularly polarized beams produced from the optical beam; and determining the unknown wavelength of the optical beam from the determined phase shift and the difference in the optical path lengths.
According to one embodiment, a difference in said optical path lengths for said reference beam can be monitored during the duration of a measurement, and said optical path lengths updated. This enhances the thermal and environmental stability of the wavemeter.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.